Forecasting California’s Earthquakes—What Can We Expect in the Next 30 Years? - "For the far northwestern part of the State [of California], a major source of quakes is the offshore 750-mile-long “Cascadia Subduction Zone,” which extends south about 150 miles into California. For the next 30 years there is a 10% probability of a magnitude 8 to 9 quake somewhere along the zone—such quakes occur about every 500 years." - USGS Fact Sheet 2008-3027, p. 4.